This invention relates to a photovoltaic device such as a solar cell and photo detector.
Recently, a method has been proposed which mass-produces low-priced solar cells by using amorphous silicon (hereinafter referred to as an "a-Si") as a semiconductor material. The a-Si is formed by decomposing, for example, silane or fluoro-silane in a glow discharge. Since the average density of the localized states of the a-Si in the forbidden band is low, below 10.sup.17 eV.sup.-1 cm.sup.-3, a shift of the Fermi-level is readily effected by doping an impurity into the a-Si structure. For this reason, the a-Si is preferably used as a semiconductor material for solar cells.
FIG. 1 is an energy band diagram of a p-i-n type solar cell comprised of a-Si. In the diagram, as shown in FIG. 1, regions 11, 12 and 13 show p-, i- and n-type layers. Level 14 is the Fermi-level and gaps 15, 16 and 17 are the optical forbidden band gaps (hereinafter referred to as "Eg's") of these layers 11, 12 and 13, respectively. Gap 18 represents a diffusion potential.
When light is incident on the p-layer, electron-hole pairs are produced in a depletion layer of the i-type layer and migrate in opposite directions due to the diffusion potential, causing them to be captured by a pair of opposite electrodes to produce an electromotive force.
In a solar cell of this type, the thicker the p-type layer, the higher the diffusion potential of the p-i junction becomes, thus increasing the open circuit bolt Voc. However, if the p-type layer is too thick, the recombination of carriers in the p-type layer is increased and the amount of light reaching the i-type layer is decreased, causing a decrease in the short circuit current density Jsc. The thickness of the p-type layer is normally at most 200 .ANG.. The thickness of the i-type layer permitting the production of the electron-hole pairs is designed to have a value of, for example, about 5000 .ANG. which substantially corresponds to the sum of the value of the width of the diffusion potential region and the diffusion length of the minority carriers. It is important that, since the region permitting the generation of photocurrent exists mostly in the i-type layer, the absorption of light in the p-type layer be decreased. A method has been considered which expands the Eg of the p-type layer, thereby permitting the entry of light. The Eg of the p-type layer can be made above 1.8 eV, for example, by controlling the conditions under which the glow discharge occurs, for example, in the formation of an a-Si layer structure. Where the p-type layer having such an Eg level is used, it is impossible to obtain an adequate open circuit voltage Voc unless the thickness of the p-type layer is made 200 to 300 .ANG. or more. The reason is that with an increase in Eg the activation energy of the a-Si layer structure increases, thereby causing the diffusion potential to decline.
FIG. 2 is a graph showing the relationship of the thickness of the p-type layer to the Voc. In FIG. 2, the curve a denotes a p-type layer of a smaller Eg level obtained by the decomposition of a 1.0 molar % B.sub.2 H.sub.6 -bearing SiH.sub.4 in a glow discharge and the curve b denotes a p-type layer of a greater Eg level obtained by the decomposition of a 0.05 molar % B.sub.2 H.sub.6 -bearing SiH.sub.4 in a glow discharge. From FIG. 2 it is evident that an in adequate voltage Voc is obtained unless the p-type layer has a greater thickness.
It is possible that the absorption of light in the p-type layer is decreased by reducing the thickness of the p-type layer while making the Eg level lower. In this case, however, a non-uniform a-Si layer structure is formed and the formation of the pi junction is inadequate, lowering the fill factor of the solar cell.